In accordance with the present state of the art, continuous laser radiation with an almost diffraction-limited beam quality can be obtained by individual laser diodes with small output powers (i.e., <1 W). Power sealing by coherent coupling of many diodes is possible for small output powers.
Solid state lasers that are pumped with laser diodes are currently used to produce higher fundamental mode powers with good electro-optical efficiency (i.e., 10-30%) in a spectral range around 1 μm. Pumping sources of reasonable brilliance can already realize an efficient fundamental mode operation by using laser transitions with small thermal occupation of the lower laser energy level (so-called four-level systems) and sufficient durability of the upper laser energy level and sufficiently large absorption and emission cross-sections. One decisive criterion in this connection is that only small thermo-optical effects (i.e., “thermal lens”) occur in the laser-active medium. When laser-active media of rod-shaped or cuboid geometries are used, this is guaranteed only up to approximately 5 to 20 W output power per laser-active element. For fundamental mode powers of more than approximately 20 W per crystal, a rod laser can produce fundamental mode operation only with reduced efficiency and reduced dynamic stability range due to thermo-optical effects. With thoroughly homogenized pump light distribution, a side-pumped Nd:YAG rod achieves a maximum fundamental mode power of around 80 W with an optical-optical efficiency of approximately 20%.
It is possible to obtain a fundamental mode power of more than 100 W from a single laser-active medium having good efficiency (i.e., >40% optical-optical) by means of a laser-active medium of optimized geometry. Either a disc laser or a fiber laser are suitable for this purpose. The thermal-optical effects of the disc laser are considerably reduced due to its axial thermal flow, parallel to the resonator radiation, and its extremely short optical path, while the fiber laser is insensitive to thermal-optical effects due to wave guidance of the laser radiation. Both geometries require a relatively high pump radiation brilliance; a typical value for a Yb:YAG disc laser is approximately 5 kW/(mm·rad)2, whereas a unilaterally end-pumped fiber laser with 100 W output power, a typical pump core diameter of 0.4 mm and a numerical aperture of 0.45 requires approximately 20 kW/(mm·rad)2. The output power is scaled in the disc laser with approximately constant pump radiation brilliance, wherein the diffraction-limited beam quality can be maintained with good efficiency only up to approximately 200 W. The output power of the fiber laser can be doubled to approximately 200 W through pumping at both sides with the same pump light brilliance. The output power of the fiber laser can be further increased, and an increase in output power requires a proportional increase of the pump light brilliance if the diameter and the numerical aperture of the pump core are kept constant. Thus, high-power fiber laser with output powers of more than 500 W and an almost diffraction-limited beam quality put very high demands on the brilliance of the pump light source(s). The requirements with regards to brilliance can be reduced by increasing the numerical aperture of the pump core by increasing the pump core diameter.
Conventional high-power fiber lasers are end-pumped with laser diode arrays having complex beam shapes and a high brilliance. The wavelength of the pump light is usually about 980 or 915 nm. The required pump light brilliance increases with increasing output power, which also increases the expense for shaping the beams of the laser diodes. High-power laser diode arrays (“stacks”) with good fast-axis collimation have a brilliance of approximately 10-20 kW/(mm·rad)2 such that further measures are used to increase the brilliance of a diode-pumped fiber laser with 1 kW output power by approximately five to ten times. For example, the further measures can include polarization coupling, slow-axis collimation, and wavelength multiplex. Due to the required multi-stage optical transformations, the electro-optical efficiency of these pump light sources with high brilliance is considerably smaller than that of a simple pump light source. At the same time, the required optical expense greatly increases the costs of the pump light sources.